1. Field of Endeavor
The present invention relates to optics and more particularly to the reduction of damage in optics.
2. State of Technology
The abstract of U.S. Pat. No. 3,999,865 for method and apparatus for determining the mechanism responsible for laser-induced damage by Milam, et al, issued Dec. 28, 1976, provides the following description, xe2x80x9cA system is described for determining the mechanism responsible for laser-induced damage in a sample which utilizes a procedure of sequentially irradiating a large number of sites using a tightly focused laser beam whose intensity is constant in time. A statistical analysis of survival times yields a determination that damage was due to one of the following mechanisms: 1. linear absorption, 2. nonlinear absorption, 3. absorbing inclusions, 4. mechanical defects, or 5. electron-avalanche breakdown.xe2x80x9d
The abstract of U.S. Pat. No. 4,667,101 for predicting threshold and location of laser damage on optical surfaces by Wigbert Siekhaus, patented May 19, 1987 provides the following description, xe2x80x9cAn apparatus useful in the prediction of the damage threshold of various optical devices, the location of weak spots on such devices and the location, identification, and elimination of optical surface impurities comprising, a focused and pulsed laser, an photo electric detector/imaging means, and a timer. The weak spots emit photoelectrons when subjected to laser intensities that are less than the intensity actually required to produce the damage. The weak spots may be eliminated by sustained exposure to the laser beam.xe2x80x9d
The abstract of U.S. Pat. No. 5,086,352 for optical members and blanks or synthetic silica glass and method for their production by Yamagata et al, patented Feb. 4, 1992 provides the following description: xe2x80x9cThe invention relates to lenses, prisms or other optical members which are subjected to high-power ultraviolet light having a wavelength of about 360 nm or less, or ionizing radiation, particularly optical members for use in laser exposure apparatus for lithography, and to blanks for such optical members. The homogeneity of the refractive index distribution and the resistance to optical deterioration when the optical members are exposed for a long period of time to short wavelength ultraviolet light from a laser beam are improved. The optical members are made of high-purity synthetic silica glass material containing at least about 50 wt. ppm of OH groups, and are doped with hydrogen. The refractive index distribution caused by the fictive temperature distribution during heat treatment in the process of producing high-purity silica glass blanks for optical members in accordance with the present invention is offset by the refractive index distribution determined by the OH group concentration distribution or by the OH group concentration distribution and the Cl concentration distribution in the glass.xe2x80x9d
The abstract of U.S. Pat. No. 5,325,230 for optical members and blanks of synthetic silica glass and method for their production by Yamagata et al, patented Jun. 28, 1994, provides the following description: xe2x80x9cThe invention relates to lenses, prisms or other optical members which are subjected to high-power ultraviolet light having a wavelength of about 360 nm or less, or ionizing radiation, particularly optical members for use in laser exposure apparatus for lithography, and to blanks for such optical members. The homogeneity of the refractive index distribution and the resistance to optical deterioration when the optical members are exposed for a long period of time to short wavelength ultraviolet light from a laser beam are improved. The optical members are made of high-purity synthetic silica glass material containing at least about 50 wt. ppm of OH groups, and are doped with hydrogen. The refractive index distribution caused by the fictive temperature distribution during heat treatment in the process of producing high-purity silica glass blanks for optical members in accordance with the present invention is offset by the combined refractive index distribution determined by the OH group concentration distribution or by the OH group concentration distribution and the Cl concentration distribution in the glass.xe2x80x9d
The abstract of U.S. Pat. No. 5,410,428 for Optical member made of high-purity and transparent synthetic silica glass and method for production thereof or blank thereof by Yamagata et al, patented Apr. 25, 1995, provides the following description: xe2x80x9cThis invention relates to an optical member made of highly transparent, high-purity synthetic silica glass, to a method for manufacturing a blank or an optical member of such glass, and to the optical members themselves. The optical members have an absolute refractive index, nd, of 1.460 or more and a hydrogen molecule concentration of at least 5xc3x971016 molecules/cm3 uniformly distributed throughout the glass and are particularly well suited for use in apparatus in which they are exposed to a high-power laser beam such as that produced by an excimer laser.xe2x80x9d
The abstract of U.S. Pat. No. 5,472,748 for permanent laser conditioning of thin film optical materials by Wolfe et al, patented Dec. 5, 1995 provides the following description: xe2x80x9cThe invention comprises a method for producing optical thin films with a high laser damage threshold and the resulting thin films. The laser damage threshold of the thin films is permanently increased by irradiating the thin films with a fluence below an unconditioned laser damage threshold.xe2x80x9d
The abstract of U.S. Pat. No. 5,616,159 for a method of forming high purity fused silica having high resistance to optical damage by Araujo et al, patented Apr. 1, 1997, provides the following description, xe2x80x9cHigh purity fused silica glass which is highly resistant to optical damage by ultraviolet radiation in the laser wavelength of about 300 nm or shorter is produced. In particular, a fused silica optical member or blank exhibits substantially no optical damage up to 107 pulses (350 mJ/cm2) at the KrF laser wavelength region of about 248 nm, and at the ArF laser wavelength region of about 193 nm.xe2x80x9d
The abstract of U.S. Pat. No. 5,697,998 for sapphire window laser edge annealing by Daniel H. Platus et al, patented Dec. 16, 1997, provides the following description, xe2x80x9cA sapphire window is laser edge annealed using a CO2 laser spot illuminating along a path following the edge of the window so as to heat the edges to remove or reduce sub surface defects which can cause stress fractures.xe2x80x9d
The abstract of U.S. Pat. No. 5,796,523 for a laser damage control for optical assembly by John M. Hall, patented Aug. 18, 1998 provides the following description, xe2x80x9cAn optical assembly and technique for magnified viewing which includes innal damaging laser energy protection. An objective lens subassembly is positioned on the focal axis which focuses incoming light energy over an entire field of view. An optical prism accepts the focused light energy and reorients the focused light energy which is split by a beamsplitter onto a power limiter at the intermediate focal plane so that only energy under an approximate damage threshold is allowed to pass onward. The optical assembly can provide from 4xc3x97 to a 10xc3x97 power magnification with up to a 60 millimeter entrance pupil diameter with internal, multi-spectral damaging laser energy protection.xe2x80x9d
The abstract of U.S. Pat. No. 6,099,389 for fabrication of an optical component by Nichols et al, patented Aug. 8, 2000 provides the following description: xe2x80x9cA method for forming optical parts used in laser optical systems such as high energy lasers, high average power lasers, semiconductor capital equipment and medical devices. The optical parts will not damage during the operation of high power lasers in the ultra-violet light range. A blank is first ground using a fixed abrasive grinding method to remove the subsurface damage formed during the fabrication of the blank. The next step grinds and polishes the edges and forms bevels to reduce the amount of fused-glass contaminants in the subsequent steps. A loose abrasive grind removes the subsurface damage formed during the fixed abrasive or xe2x80x9cblanchardxe2x80x9d removal process. After repolishing the bevels and performing an optional fluoride etch, the surface of the blank is polished using a zirconia slurry. Any subsurface damage formed during the loose abrasive grind will be removed during this zirconia polish. A post polish etch may be performed to remove any redeposited contaminants. Another method uses a ceria polishing step to remove the subsurface damage formed during the loose abrasive grind. However, any residual ceria may interfere with the optical properties of the finished part. Therefore, the ceria and other contaminants are removed by performing either a zirconia polish after the ceria polish or a post ceria polish etch.xe2x80x9d
The abstract of U.S. Pat. No. 6,205,818 for production of fused silica having high resistance to optical damage by Thomas P. Seward III, patented Mar. 27, 2001 provides the following description, xe2x80x9cA method of rendering fused silica resistant to compaction caused by UV laser beam irradiation. The method of the invention results in a fused silica member that is desensitized to compaction caused by the long-term exposure to UV laser beams. The invention includes a means to pre-compact fused silica members using high energy radiation.xe2x80x9d
The present invention provides a method of reducing the initiation of catastrophic damage on the surface of fused silica optics by conditioning the optic at low fluences below levels that normally lead to catastrophic growth of damage. When the optic is then irradiated at its high fluence design limit, the concentration of catastrophic damage sites that form on the surface of the optic is greatly reduced. The present invention provides a method of conditioning fused silica optics to reduce damage when said fused silica optics is operated in an environment where it will be exposed to high-power ultraviolet light. The fused silica optics is irradiated in successive irradiating steps in ramp-like fashion. The fused silica optics is irradiated with successive irradiating steps applied in increasingly higher fluences. In one embodiment of the present invention, the step of irradiating the fused silica optics with a conditioning laser beam of about 360 nm wavelength or less is conducted with a conditioning laser beam with a wavelength in the range of 360 nm to 150 nm. In another embodiment of the present invention the step of irradiating the fused silica optics with a conditioning laser beam of about 360 nm wavelength or less is conducted with a conditioning laser beam of approximately 355 nm. In another embodiment of the present invention the fused silica optics is conducted with substantially less than the fluence that could cause more than half the density of catastrophic damage sites in an unconditioned optic. In another embodiment of the present invention successive irradiating steps are continued until the density of catastrophic damage on the fused silica optics is expected to be reduced by at least a factor of 2.
A feature of the invention is to reduce of initiation of catastrophic damage on the surface of fused silica optics, such that the optics can survive prolonged exposure to high-power laser beams having an ultraviolet wavelength of about 360 nm or less.
Another feature of the invention is to substantially improve the resistance to optical deterioration of already manufactured optical components made of fused silica, such that these optical components can survive prolonged exposure to high-power ultraviolet irradiation. The invention enables such improvement in the optical components with minimum or no change in the manufacturing process for said optical components.
Another feature of the invention is to substantially improve the resistance to optical deterioration of high-energy laser components made of fused silica for applications to UV/DUV/EUV lithography, especially in semiconductor manufacturing.
Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description and by practice of the invention.